1. Field of the Invention
The present invention relates to a thermal fixing device that fixes toner onto recording sheets and to an image forming device provided with the thermal fixing device.
2. Description of the Related Art
Conventional image forming devices such as facsimile machines and copy machines are known for forming toner images onto a recording sheet. The image forming devices include a transfer unit and a thermal fixing device. The transfer unit transfers the toner image onto a recording sheet and the thermal fixing device fixes the toner onto the recording sheet.
The thermal fixing device includes a heat roller and a pressure roller. The thermal fixing device fixes toner onto sheets using the operation of this pair of rollers. That is, the heat roller includes an internal heater and serves to melt the toner onto the recording sheet. The pressure roller sandwiches the recording sheet between itself and the heat roller and presses the toner onto the recording sheet. The heat roller and the pressure roller transport the recording sheet from upstream to downstream by rotating in mutually opposite directions while the recording sheet is sandwiched therebetween.
Often toner is poorly fixed on recording sheets because the thermal fixing device is used to fix toner onto recording sheets before heat from the heat roller has properly accumulated in the pressure roller. This is particularly a problem when the image forming device is located in a cold room, because the recording sheets supplied to the image forming device will also be cold and draw a great deal of heat from the pressure roller.
To overcome this problem, Japanese Patent-Application Publication No. 5-127565 discloses a thermal fixing device that performs not only an initial warm up, but also a high-temperature warm up for a fixed time after the initial warm up. During the high-temperature warm up, the fixing roller is heated up to a higher temperature than the normal fixing temperature used to fix toner onto recording sheets, so that the thermal fixing device properly fixes toner to sheets.
Japanese Patent-Application Publication No. 2-211472 discloses a thermal fixing device wherein if the heat roller is heated up to a predetermined temperature within a fixed time, then printing is performed immediately. On the other hand, if it takes longer than the predetermined time, printing is delayed for a fixed duration of time wherein the radiant heat of the heat roller warms up the surrounding components so that printing can be performed without influence from the ambient temperature.
Japanese Patent-Application Publication No. 3163483 discloses a thermal fixing device that determines whether the surface temperature of a fixing roller reaches a set temperature within a fixed time period from when power is turned ON. When it takes longer than a fixed duration of time, then this means that the surrounding environment is cool or applied voltage is insufficient. In this case, the fixing roller is heated until it reaches a higher temperature than used when the surrounding environment is warm or applied voltage is sufficient.